Hi Maggie,
I've looked in the Thame PRs (Oxfordshire FHS CD) and there's quite a bit about this baptism so I'll copy out what it says:
"baptised May 4 1791 James Clements s/o **John & Ann from London.
On Folio 102 of the first register, there is a note as follows:-
The above is a true extract from the Register Book of Baptisms belonging to the parish of Thame in the county of Oxford taken this 27 day of October 1838 by me T T Lee - vicar.
I received the following on 2nd day of April 1839 of Mr Clemmans - TT Lee, vicar.
James John Clemmans of Holywell Row in the parish of St Leonard, Shoreditch, London called upon Revd TT Lee of the parish of Thame, Oxfordshire on Tuesday 2nd April 1839 and stated that he is the person to whom the above Register refers and that the above was registered in error as he verily believes and of which he is informed by Mr John Howlett of this parish who was present at his baptism.
Witness: Thos Howlett, Dinton, Bucks & John Howlett of Thame, Oxon."
[** definitely says John not James as the father]
Make of that what you will!! Pity it doesn't make any attempt to say where he WAS baptised!! Intriguing!!
There are no Clements (or any spelling variation) marriages in Thame between 1601 and 1837, and just one Heaton marriage which is probably not relevant:
Oct 8 1782 David Heaton of the parish of Brading in the Isle of White [sic] & X Elizabeth Howlett of this parish. Witnesses: John Hunt, Sarah West .
Jill